1. Field of the Invention
The present invention relates generally to a method and apparatus for retrieving subterranean core samples under pressure and, more specifically to a method and apparatus for recovering core samples under insitu pressure and temperature.
2. Background of the Invention
The recovery of subterranean, geologic samples is commonly performed by an operation or technique referred to as coring. This technique has evolved from simple single tube systems to dual tube systems that are most commonly used in the mining and petroleum industry today. Because such coring techniques are employed for recovery of volatile components contained within rock samples, various modifications have been made to conventional coring devices in order, for example, to retain formation pressure on the core during recovery.
In order to accurately analyze the composition of certain volatile core samples, the core sample must maintain its chemical, mechanical, and/or physical integrity during the retrieval process. Downhole, water or other substances in the formation may contain dissolved gases which are maintained in solution by the extreme pressure exerted on the fluids when they are in the formation. Thus, unless a pressure core barrel is employed during the core extraction process, the pressure on the core at the surface will differ dramatically from the pressure experienced on the core sample downhole. Furthermore, as the pressure on the core sample decreases, fluids in the core will expand and any gas dissolved therein will come out of solution. Accordingly, the retrieved core sample will not accurately represent the composition of the downhole formation.
One common method of retaining core integrity is known as pressure coring. Pressure coring utilizes various apparatuses to maintain the core sample at or near formation pressure as the core is retrieved to the surface. Core sampling tools that include pressurized core barrels have been known for several decades. For example, U.S. Pat. No. 2,248,910 to D. W. Auld et al. entitled "PRESSURE RETAINING CORE BARREL" discloses a core barrel that is sealed downhole to maintain the core at downhole pressure. U.S. Pat. No. 3,548,958 to Blackwall et al. discloses another pressure core barrel that utilizes a compressed gas system to maintain pressure on the core sample during the core retrieval process. U.S. Pat. No. 4,317,490 to Milberger et al. discloses yet another pressurized core barrel in which a ball valve, actuated from the surface is employed to trap ambient pressure in the core barrel while downhole. U.S. Pat. No. 4,466,495 to Jageler discloses a pressure core barrel of a sidewall coring tool. Other pressure core barrels are disclosed in U.S. Pat. No. 4,356,872 to Hyland, U.S. Pat. No. 4,256,192 to Aumann, the inventor of the present invention, U.S. Pat. No. 4,230,192 to Pfannkuche, U.S. Pat. No. 4,142,594 to Thompson et al., U.S. Pat. No. 4,014,393 to Hensel, Jr., and U.S. Pat. No. 4,735,269 to Park et al. Pressure core barrels often utilize pressure actuation to release a latch and/or mechanical manipulation of the drill pipe to close a valve and also often require the entire core barrel to be brought to the surface to recover the core.
Encapsulation is another technique known in the art to maintain the integrity of unconsolidated or friable core samples. In U.S. Pat. No. 4,449,594 to Sparks, a foam is introduced into the well under a correlated control pressure. The core sample is thus encapsulated while the reservoir pressure within the sample is balanced by the bottom hole foam balance pressure to produce a balanced, pressurized core sample. Another method of encapsulating a core sample is disclosed in U.S. Pat. No. 4,716,974 to Radford et al. in which a liquid foam is allowed to cure to form a sponge-like solid that retains oil as the core is depressurized during retrieval. Another attempt to stabilize cores where unconsolidated and friable columnar masses of earth can be handled without altering the characteristics of its physical structure employs a rubber sleeve that encapsulates the core sample. A housing is provided for positioning the ensleeved core therein and subfreezing material is circulated around the ensleeved core to freeze and solidify the core fluids contained therein. Likewise, in U.S. Pat. Nos. 5,360,074, 5,560,438, 5,546,798, and 5,482,123 to Collee et al., methods for maintaining the mechanical integrity and for maximizing the chemical integrity of a core sample during transport from a subterranean formation to the surface comprises employing an encapsulating material that increases in viscosity or even solidifies at temperatures slightly lower than those expected downhole. The patents to Collee note that in such a method of encapsulation, the chemical integrity of the core sample can be further increased by using a pressure core barrel.
Certain core samples, however, such as cores containing methane hydrate, not only require that the core sample be maintained at formation pressure when brought to the surface for examination and testing, but because methane hydrate is a material stable only within a limited pressure/temperature range, the core sample must also be maintained at formation temperature during recovery. If the core sample is allowed to become heated above this pressure/temperature envelope during the extraction process, the structural and physical makeup of the sample will be partially if not totally lost.
One attempt in the art to retrieve methane hydrate cores is disclosed in U.S. Pat. No. 4,371,045 to McGuire et al. As described, the cores are cooled down to at least -80 degrees C. at which temperature the pressure of methane hydrates is 1 atmosphere. Such cooling is accomplished by employing a conventional wire line retrievable core barrel having perforations therein through which cryogenic liquid passes into direct contact with the hydrocarbon hydrates and thus thermodynamically stabilizes the core. The invention employs an insulated chilling vessel into which the perforated core barrel and thus the core sample is moved for cryogenic freezing.
Many of the aforementioned coring apparatuses employ valves or other sealing devices to isolate the core. For example, a common method of preventing fluid access to the inner tube of a core barrel assembly is provided in U.S. Pat. No. 5,230,390 to Zastresek et al. in which a closure mechanism is configured to move from an open condition to a closed condition in response to increased fluid flow rates and pressure differentials occurring at the closure mechanism. Likewise, U.S. Pat. No. 5,253,720 to Radford et al. discloses a coring device in which a ball valve is actuated to seal off the core barrel before the core barrel is pulled to the surface.
It is also noted, that wire line retrieval of core barrels and/or manipulation of various components of the coring apparatus has previously been employed in many of these systems. For example, in U.S. Pat. No. 3,627,067 to Martinsen, a core-drilling system is disclosed in which selective or controlled release of an overshot from the core barrel while downhole is performed by pumping a wire line to which the overshot is attached up and down a prescribed number of times. In U.S. Pat. No. 3,667,558 to Lambot, an upward pull on a cable unlatches the coring head and also vents water under pressure so that it no longer forces the assembly downward. Continued pulling on the cable retrieves the coring head and the core sample. U.S. Pat. No. 3,739,865 to Wolda, discloses a wire line core barrel system that includes flexible latch fingers and provides a predetermined pressure signal indicating latching and further blocks fluid flow until the core barrel is properly latched. U.S. Pat. No. 4,800,969 discloses yet another wire line core barrel assembly in which an inner tube assembly can move down faster than the fluid flow in the drill stem during the time the inner tube assembly moves downwardly in the drill stem. U.S. Pat. No. 4,466,497 to Soinski et al. discloses yet another wire line core barrel apparatus.
Other coring systems and devices are known such as the coring apparatus disclosed in U.S. Pat. No. 3,874,465 to Young et al. in which core samples of relatively soft formations may be retrieved. The coring apparatus comprises a core barrel with an interior surface having properties similar to synthetic rubber, two semi-tubular rigid portions joined along the adjacent edges by a flexible material, and a core catcher having a plurality of flexible segments adapted to open while the core is being drilled and to close when the core is to be recovered. A latch for retaining the tool in position within the coring bit and a swivel allowing the core barrel and catcher to remain stationary while the coring bit is rotated are also provided.
While the aforementioned references disclose various methods and apparatuses for retrieving core samples of subterranean formations, these methods are inadequate to maintain a core sample at least partially comprised of methane hydrate at its downhole state. U.S. Pat. No. 4,371,045, which is specifically directed to the problem of stabilizing hydrocarbon cores, requires that the core be quickly brought to the surface before cryogenic freezing of the core is performed. Thus, it would be advantageous to provide a method and apparatus for retrieving core samples that are or become unstable when removed from the downhole environment. Such a coring method and apparatus may be applicable to not only obtaining core samples of formations containing hydrocarbons, but may have utility in other coring applications where the core samples may be unconsolidated, friable, or comprised of frozen material that would otherwise not maintain their chemical or mechanical properties once exposed to ambient pressures and temperatures. In addition, the methods and apparatuses disclosed herein may have applicability to other coring devices regardless of the type of formation from which the core sample is being taken.